In a display system, one or more light sources are driven by a driving circuit for illuminating a display panel. For example, in a liquid crystal display (LCD) display system with light-emitting diode (LED) backlight, an LED array is used to illuminate an LCD panel. An LED array usually includes two or more LED strings, and each LED string includes a group of LEDs connected in series. For each LED string, the forward voltage required to achieve a desired light output may vary with LED die sizes, LED die material, LED die lot variations, and temperature. Therefore, in order to generate desired light outputs with a uniform brightness, driving circuits are used to regulate the current flowing through each LED string to be substantially the same.
FIG. 1 shows a block diagram of a conventional LED driving circuit 100. The LED driving circuit 100 includes a DC/DC converter 102 for converting an input DC voltage VIN to a desired output DC voltage VOUT for powering LED strings 108_1, 108_2, . . . 108—n. Each of the LED strings 108_1, 108_2, . . . 108—n is respectively coupled to a linear LED current balance controller 106_1, 106_2, . . . 106—n in series. A selection circuit 104 receives monitoring signals from current sensing resistors RSEN_1, RSEN_2, . . . RSEN_N and generates a feedback signal. The DC/DC converter 102 adjusts the output DC voltage VOUT based on the feedback signal. Operational amplifiers 110_1, 110_2, . . . 110_N in the linear LED current balance controllers compare the monitoring signals from current sensing resistors RSEN_1, RSEN_2, . . . RSEN_N with a reference signal REF respectively, and generate control signals to adjust the resistance of transistors Q1, Q2, . . . QN respectively in a linear mode. In other words, the conventional LED driving circuit 100 controls transistors Q1, Q2, . . . QN linearly to adjust the LED currents flowing through the LED strings 108_1, 108_2, . . . 108_N respectively. However, this solution may not be suitable for systems requiring relatively large LED current because of the larger amount of heat generated by the transistors Q1, Q2, . . . QN. As such, the power efficiency of the system may be decreased due to the power dissipation.
FIG. 2 shows a block diagram of another conventional LED driving circuit 200. In FIG. 2, each LED string is coupled to a dedicated DC/DC converter 202_1, 202_2, . . . 202_N respectively. Each DC/DC converter 202_1, 202_2, . . . 202_N receives a feedback signal from a corresponding current sensing resistor RSEN_1, RSEN_2, . . . RSEN_N and adjusts an output voltage VOUT_1, VOUT_2, . . . VOUT_N respectively according to a corresponding LED current demand. One of the drawbacks of this solution is that the system cost can be increased if there are a large number of LED strings, since a dedicated DC/DC converter is required for each LED string.